The today known operative processes for producing stable aqueous solutions of polyurethanes are various; in literature there is a large patent and bibliographic documentation, see for instance D. Dieterich, Progress in Organic Coatings, 9, 281 (1981).
Such aqueous dispersions are nearly all of isomeric type, i.e. they contain inside the polyurethanic macromolecule some hydrophile centers of ionic type, which act as internal surfactants and make the polymer dispersable or soluble in water.
The introduction of such ionic centers is carried out by employing particularly diols containing groups which can be salified and which do not react or react with difficulty with the isocyanic groups.
The preparation methods may be of solvent type, that is to say the polyurethane, before its dispersion in water, is obtained in solution of an organic solvent, that is inert towards the free isocyanic groups, such as acetone, methylketone, dioxane, tetrahydrofuran, which, at the end, are removed by distillation.
The amount of organic solvent is generally very high, from 1 to 3 times the weight of the polyurethane, considered as dry, therefore it is obvious that such a process presents some drawbacks, when it is applied on industrial scale, because of the low productivity, the necessity of recycling the solvent, the distillation operations and the long preparation time owing to the low temperature which can be reached.
Methods which allow a sensible reduction in the organic solvent are described in the patent literature: for instance in U.S. Pat. No. 4,183,836 a polyisocyanic polymer containing carboxylic groups as well, is dispersed in an aqueous solution containing a tertiary amine, that acts as salification agent, and a diamine that acts as chain lengthening agent, by acting on the isocyanic terminal groups of the prepolymer, thereby obtaining an anionic polyurethane-polyurea dispersed in water.
In German Pat. No. 2,725,589 a polyisocianic prepolymer containing ionic groups is mixed, under anhydrous conditions, with diamines blocked by ketones (ketonimines) which result to be inert towards the isocyanic groups; the subsequent addition of the water hydrolyzes the ketoniminic groups to aminic group, therefore the free diamine is formed again, that acts as chain lengthening agent.
In European Pat. No. 4069 the lengthening (elongation) of the polyisocyanic prepolymers containing ionic groups is carried out after mixing, under anhydrous conditions, with hydrazines blocked by ketones (ketonhydrazines) and subsequent addition of water.
In U.S. Pat. No. 3,756,992 another preparation method is described, according to which a polyisocyanic prepolymer containing ionic and/or ionizable centers is previously turned into a prepolymer with ureic terminal groups (--NH--CO--NH.sub.2) by reaction with urea, by working at high temperature in order to maintain the prepolymer in the method state; after its dispersion in water, on such a prepolymer, methylolic groups are introduced on the ureic terminal groups or also on the NH-groups of the preformed urethanic groups, by means of formaldehyde; by subsequent self-condensation of the formed methylolic groups, polyurethanes with a high molecular weight are obtained.
By using methods based on the water dispersion of ionized and subsequently lengthened (extended) polyisocyanic prepolymers with free or differently blocked diamines and/or hydrazines, essentially linear polyurethanes-polyureas are obtained and such methods can be easily applied by employing polyisocyanic prepolymers based on aliphatic isocyanates; on the contrary, with aromatic prepolymers the high reactivity of the isocyanic groups towards the water as well, involves problems of foaming of the bulk and of formation of coagula, which cannot be dispersed again.
On the other hand, the methods based on the condensation of the ionomeric methylated polyuirethanes, although they allow to obtain reticulated structures and, consequently, films endowed with high resistance towards the solvent, present the drawback to give rise to finished products, which always contain traces of free formaldehyde, with a very sharp stink and, above all, to release to the environment amounts of formaldehyde previously linked as methylolic or methylo-ether group, besides the free one, during the applications steps, which, nearly all of them, foresee a thermic treatment.